1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor having photosensitivity in the long wavelength range up to about 800 nm which can be applicable as a photoreceptor for a semiconductor laser beam printer and to a method for making the same.
2. Prior Art
Electrophotography is a process comprising charging a photoreceptor, imagewise exposing to provide a latent electrostatic image, developing with a developing agent, then converting it to a toner image and fixing so as to obtain a duplicate. The photoreceptor used in the electrophotography consists basically of a light sensitive layer composed of a photoconductive layer formed on an electroconductive substrate. As the materials forming the photoconductive layer, an inorganic photoconductive material such as selenium or alloys thereof, cadmium sulfide or zinc oxide, or an organic photoconductive material such as polyvinyl carbazole, trinitrofluorenone, bisazo pigments, phthalocyanine, pyrazoline or hydrazone are known. The photoconductive layer may comprise one layer or a plurality of layers laminated.
Photoreceptors using amorphous silicon as a photoconductive layer have recently been developed, and various improvements have been attempted. The photoreceptor using the amorphous silicon consists of a conductive substrate on which is an amorphous silicon film is formed by e.g. glow-discharge decomposition of silane (SiH.sub.4), and the hydrogen atoms are trapped in the amorphous silicon film to show the photoconductivity. The amorphous silicon light sensitive material has a high surface hardness of the photoconductive layer layer, a high resistance to scratching, wear and a high temperature, a high mechanical strength, and a high light sensitivity.
However, though the above amorphous silicon photoreceptor has high photosensitivity in the wavelengths range of about 400 to 700 nm, the light absorbability decreases in the longer wavelengths equal or more than 700 nm and the light sensitivity decreases radically.
Modern laser beam printers using semiconductor lasers as light sources require electrophotographic photoreceptors which have high photosensitivity in the longer wavelength range up to 800 nm. The above amorphous silicon photoreceptor however could not satisfy the requirement and thus could not be rendered for practical use for a semiconductor laser printers. Therefore, amorphous silicons containing germanium have been suggested as longer wavelength sensitization methods (Japanese Patent Applications (OPI) No. Sho 57-115552, Sho 58-171043 and Sho 61-243461). In addition, the doping of amorphous silicon germanium photoreceptor with boron was suggested in the 49th Research and Discussion meeting of the Electrophotographical Society (1982).
However, though high sensitivity and high dark resistance are expected as required characteristics of electrophotographic photoreceptor, the photoreceptor sensitized in the long wavelength has low dark resistance and displays special exhausted effects. Such light exhaustion leads to a decrease of the large density and a occurance of ghost image to deteriorate the image quality.
In addition, the charge injection blocking layer formed between the substrate and the photoconductive layer is required to avoid the injection of charges having the opposite polarity to the polarity of the electrification. On the other hand, the charges having the same polarity as the polarity of the electrification are required to flow toward the subtrate at the time of irradiation. Thus it can be imagined that in general a p-type layer for positive electrification and a n-type layer for negative electrification may be formed in an amorphous silicon photoreceptor. However, if a layer in which the polarity has been changed by doping amorphous silicon hydrogenated with a high content of a group III element or a group V element, the adhesive characteristics with the substrate or with the photoconductive layer formed on the substrate become worse. Therefore, a layer containing carbon, nitrogen or oxygen as a third element is formed heretofore.
However, if these third elements are contained, the charge injection blocking capability is insufficient, in particular it tends to be in sufficient in a high electric field, there occurs a problem that the electrification potential is easy to fall due to the repeated electrification in the dark and that the residual potential occurs.